One of the most significant innovations in the past half century has been the introduction of the rotating rigid-disk for digital data storage. In the conventional rigid-disk drive system, a transducer is supported on an air bearing slightly above the rotating disk medium. This system provides high reliability and a high data rate, and also enables fast access of data stored on the disk. The popularity of rigid-disk systems has been so great that it is now the overwhelming design of choice for on-line storage of computer data.
One of the primary requirements of a hard disk drive system is the need to maintain the spindle motor speed of the drive at a constant velocity. This generally requires the application of a steady-state current to the spindle motor, and some associated feedback correction. The feedback correction is usually in the form of a standard control loop or servo mechanism. One of the problems that arises in such a system is that the heads apply a drag force to the spindle motor. This drag force results from the interaction between the head and air that circulates around the disks as the disks rotate at high velocity. For example, modern disk drive systems typically include spindle motors which rotate at velocities of 5400 rpms or greater. Because the heads are in such close proximity to the disk surface, a frictional drag is generated on the disk due to windage.
The specific nature of the problem that arises involves the fact that the drag force on the spindle motor changes with the position of the heads from the inside diameter to the outside diameter of the disk. This is believed to be due to the fact that the velocity of the air circulating around the disk is greater at the outside diameter of the disk than at the inside diameter. As the actuator moves the heads across the disk surface, the drag induces torque disturbances which are translated into disk speed errors. Obviously, these disk speed errors must be compensated out by the control loop or servo feedback loop of the disk drive. However, this involves an associated settling time for each torque disturbance induced on the motor. Such torque disturbances are not desirable because the format of the disk requires that a certain tolerance be placed on the spindle speed accuracy. Spin errors must be accounted for in the format by leaving unused disk space. This unused disk space is wasted because it cannot be used to store data. This problem is especially acute in small form factor disk drives (e.g., drives having a disk diameter of 3.5" or less) since the torque disturbance is generally great relative to the inertia and torque of the disk assembly.
What is needed then is some way of precompensating the disk drive system for the torque disturbance which occurs as the heads traverse the disk surface in a radial direction. As will be seen, the present invention provides an apparatus and method for solving this problem by application of a predetermined torque correction value to the spindle motor during movement of the actuator.